The diffuse γ-ray background is dominated by star-forming galaxies

• Published in: Nature (London) 2021-09, Vol.597 (7876), p.341-344
• Main Authors: Roth, Matt A., Krumholz, Mark R., Crocker, Roland M., Celli, Silvia
• Format: Article
• Language: English
• Subjects: 639/33/34/863; 639/33/34/864; 639/33/34/866; Active galactic nuclei; Atoms & subatomic particles; Cosmic radiation; Cosmic ray showers; Cosmic rays; Emissions; Energy; Fermi Gamma-ray Space Telescope; Galaxies; Gamma emission; Gamma rays; Humanities and Social Sciences; Interstellar matter; Interstellar medium; multidisciplinary; Science; Science (multidisciplinary); Space telescopes; Star & galaxy formation; Star formation; Stars; Stars & galaxies; Supernova; Supernova remnants; Supernovae
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-03802-x

The Fermi Gamma-ray Space Telescope has revealed a diffuse γ-ray background at energies from 0.1 gigaelectronvolt to 1 teraelectronvolt, which can be separated into emission from our Galaxy and an isotropic, extragalactic component 1 . Previous efforts to understand the latter have been hampered by the lack of physical models capable of predicting the γ-ray emission produced by the many candidate sources, primarily active galactic nuclei 2 – 5 and star-forming galaxies 6 – 10 , leaving their contributions poorly constrained. Here we present a calculation of the contribution of star-forming galaxies to the γ-ray background that does not rely on empirical scalings and is instead based on a physical model for the γ-ray emission produced when cosmic rays accelerated in supernova remnants interact with the interstellar medium 11 . After validating the model against local observations, we apply it to the observed cosmological star-forming galaxy population and recover an excellent match to both the total intensity and the spectral slope of the γ-ray background, demonstrating that star-forming galaxies alone can explain the full diffuse, isotropic γ-ray background. The diffuse, isotropic background of gamma rays comes mainly from star-forming galaxies, according to a physical model of gamma-ray emission.